To supply fluid under pressure discharged by a single hydraulic pump into a plurality of hydraulic actuators, it is only necessary to provide a plurality of operating valves and construct a hydraulic circuit system so as to supply pressurized fluid into each of the hydraulic actuators by switching over the operating valves. In such a circuit system thus constructed, when pressurized fluid is supplied into a plurality of actuators at the same time, pressurized fluid is supplied only into hydraulic actuators with low loading thereon, but not supplied into those with high loading thereon.
As a hydraulic circuit system which overcomes this disadvantage, the system shown, for example, in the publication of Japanese Patent No. HEI 2-49405 has been proposed.
A schematic diagram of such a hydraulic circuit system is shown in FIG. 7.
Stating in brief, a hydraulic pump 1 has a discharge conduit 1a which is provided with a plurality of operating valves 2, and pressure compensating valves 5 are provided in circuits 4 connecting the operating valves 2 with hydraulic actuators 3, respectively. The arrangement is made such that when the operating valves 2 are operated at the same time the hydraulic actuators 3 can be supplied with pressurized fluid at a flow-rate distribution ratio proportional to the ratio in the area of openings of operating valves 2 by detecting the pressure in each of the circuits 4, that is, a highest value of the load pressures by means of check valves 6, and applying the detected pressure to each of pressure compensating valves 5 to set it at a pressure corresponding to the load pressure, thereby equalizing the pressures on the outlet sides of the operating valves 2.
In such a hydraulic circuit system, a flow rate distribution of pressurized fluid proportional to ratio in the area of openings of the operating valves 2 is obtained by the functions of the pressure compensating valves 5 irrespective of the magnitude of loading on each hydraulic actuator 3 so that the pressurized fluid discharged by the single hydraulic pump 1 can be supplied into the hydraulic actuators 3 at a flow-rate distribution ratio proportional to the ratio in the manipulated variables of the operating valves 2.
However, since the load pressures on the hdyraulic actuators 3 are detected and compared on the outlet sides of the pressure compensating valves 5, and then a highest pressure of the load pressure is introduced into pressure receiving portions 5a which serve to increase the setting pressure of the pressure compensating valves 5, the detected pressure Pa is lower than the inlet pressure P.sub.6 by a value corresponding to the pressure loss of fluid passing through each pressure compensating valve 5. As a result, the flow rates of pressurized fluid passing through the pressure compensating valves 5 are accompanied by an error corresponding to the pressure loss, thereby causing a flow rate distribution error.
Stating in brief, the flow rate Q.sub.1 of pressurized fluid passing through the pressure compensating valve 5 with a low load pressure thereon and the flow rate Q.sub.2 of pressurized fluid passing through the pressure compensating valve 5 with a high load pressure thereon can be expressed by the following equations. ##EQU1##
Wherein C is a constant, a.sub.1 and a.sub.2 are the areas of openings of operating valves, and Pc is the discharge pressure.
Therefore, an error corresponding to the pressure loss (Pb-Pa) through each pressure compensating valve 5 occurs in the respective flow rates.
Further, the above-mentioned problem can be solved by detecting the load pressures on the inlet sides of the pressure compensating valves 5, however, because the same pressure Pb acts on the pressure receiving portion on which a high setting pressure is applied and that on which a low setting pressure is applied, the pressure compensating valves 5 are closed by the respective springs 7 so that no pressurized fluid is supplied into the hdyraulic actuators 3.
Further, when the operating valves 2 are located at their neutral positions, the holding pressure for each hydraulic actuator 3 is supplied through the check valve 6 into a displacement control unit 8 of the hydraulic pump 1 so that the displacement of the hydraulic pump 1 is increased to raise the discharge pressure of the hydraulic pump 1 so as to correspond to the holding pressure, thereby wasting the drive horsepower developed by the hydraulic pump 1. To cope with this, if the circuit for introducing the load pressure into the displacement control unit 8 is connected through a restrictor 9 with the fluid tank so as not to increase the displacement of the hydraulic pump 1, then the holding pressure is released through the restrictor 9 to the fluid tank, thereby causing a very large spontaneous lowering of the hydraulic actuators to render it impossible to hold the latter. To eliminate this defect, the prior art hydraulic circuit is provided with a counter-balancing valve to prevent the holding pressure for each of the hydraulic actuators 3 from being led to each of the check valves 6, thus complicating the circuit arrangement and increasing the number of component parts, which results in a significant cost reduction.